This invention pertains to the art of heat pumps, and in particular to a refrigerant coil and refrigerant flow arrangement for an outdoor unit of an air-to-air heat pump.
The growing desire to conserve oil and gas has enlarged the geographic region targeted for the application of electric heat pumps to include more northerly areas. Colder air temperature requires that a northern climate heat pump should have higher efficiency than that which is available with any unit which is currently manufactured, so far as I know. This is necessary if the total cost of ownership over the first several years of operation is to be comparable with that for a gas furnace.
As is well known, when a heat pump is in the heating mode, the outdoor heat exchanger or coil removes heat from the outside air through evaporating the refrigerant passing through the coil. As is also known in this art, from a thermodynamic standpoint it is desirable to exchange heat using the smallest practicable temperature difference between the air and the refrigerant. It is also desirable that this temperature difference be as near constant as possible throughout the entire heat exchanger. When these requirements are met to the fullest extent allowed by the various design constraints, the refrigerant vapor leaves the evaporator with the highest possible temperature for a given air temperature so that the temperature lift sustained by the heat pump is minimized. Consequently, this promotes a closer approach between the system COP (coefficient of performance) and the Carnot COP. If a relatively constant refrigerant temperature throughout the coil functioning as an evaporator is to be approached, minimizing the pressure drop of the refrigerant passing through the coil is vital. The refrigerant pressure drop is increasingly punitive as outdoor temperatures are lowered because of the increasing rate of change of temperature with respect to pressure with the lowered temperatures, as is readily apparent from the saturated vapor line of the pressure-enthalpy diagram for a given refrigerant.
It is known that natural thermosyphon heat exchangers as used in some processes have an inherently low pressure drop. However, so far as I know, the concept of providing a coil with a refrigerant flow arrangement therefor which is operable in a thermosyphon manner has never been applied to an outdoor unit of a heat pump.
It is the aim of this invention to provide a particular coil disposition and refrigerant flow arrangement for an outdoor unit so that the coil operates in an optimal thermosyphon fashion in the heating mode of the heat pump.